A ~250 KYR TEPHROSTRATIGRAPHY FROM THE DEEPCHALLA DRILLED LAKE RECORD: NEW CHRONOSTRATIGRAPHIC TIE-LINES BETWEEN PALAEOENVIRONMENTAL AND GEOARCHAEOLOGICAL SEQUENCES IN KENYA
Thirty visible tephra layers derive either from off-rift basaltic scoria-cone eruptions in the nearby Chyulu Hills and Mt Kilimanjaro, from explosive activity at Mt Meru ~100 km west of Chala, or from volcanoes further afield in the Central Kenyan Rift. Direct Ar-dating of 11 tephra layers, with suitable K-rich crystal contents, provide absolutely dated anchor points for the DeepCHALLA chronology. Geochemical correlations between visible tephra found in Lake Chala sediments and in geoarchaeological sequences across Kenya demonstrate the potential for using tephra isochrons to precisely link these records. Ongoing analysis of this sediment record indicates that the visible tephra layers represent only a fraction of those recorded in the Lake Chala sequence. Also present throughout the sequence are cryptotephra – microscopic layers of tephra not visible to the naked eye that derive from extremely far-travelled, or low volume, explosive eruptions. The DeepCHALLA tephra record is providing unprecedented detail into the timing and reach of past explosive volcanism in the Kenyan Rift. Furthermore, with both regional and extra-regional tephra tie lines to geoarchaeological sequences, the site is a key locality for refining an interconnected tephrostratigraphy of the EAR.